Development of efficient, stable, and robust bifunctional electrodes is important for the practical and industrial application of water electrolysis technologies for renewable hydrogen production. Here, we report the fabrication of heterostructured Ni(OH)(2) nanosheets/Ni3S2 nanoneedles grown on Ni foam, Ni(OH)(2)/Ni3S2/NF, as a highly active and mechanically strong bifunctional electrode for overall water splitting. The results indicate that heterostructured Ni(OH)(2)/Ni3S2 interfaces are the reactive center for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The redistribution of electrons and formation of the Ni delta+ and S delta- active species serve as the hydroxyl and proton acceptors and significantly promote the transformation of intermediate species and charge transfer processes in both the OER and HER The Ni(OH)(2)/Ni3S2/NF electrode shows overpotentials as low as 252, 279, and 319 mV to reach 30, 50, and 100 mA cm(-2), respectively, for OER and 48, 79, and 185 mV to achieve 10, 20, and 100 mA cm(-2), respectively, for HER in 1.0 M KOH solution. An alkaline electrolyzer assembled by bifunctional Ni(OH)(2)/Ni3S2/NF electrodes displays cell voltages of 1.80 V to deliver 100 mA cm(-2) and is stable for over 100 h. This study demonstrates promising potential of the Ni(OH)(2)/Ni3S2/NF bifunctional electrode for advancing water-splitting technologies.